Sometimes I hear people around me at Mass saying with the priest, “Through him, with him, in him, in the unity of the Holy Spirit, all glory and honor is yours, almighty Father, forever and ever.” I was not taught to do this. Is this optional or should I say it also?
This concluding part of the Eucharistic Prayer, called the Per Ipsum, is to be said by the celebrating priest alone or with concelebrating priests, not by the faithful. Inaestimabile Donum makes this quite clear:
It is reserved to the priest, by virtue of his ordination, to proclaim the Eucharist Prayer, which of its nature is the high point of the whole celebration. It is therefore an abuse to have some parts of the Eucharistic Prayer said by the deacon, by a lower minister, or by the faithful. On the other hand the assembly does not remain passive and inert; it unites itself to the priest in faith and silence and show its concurrence by the various interventions provided for in the course of the Eucharistic Prayer: the responses to the Preface dialogue, the Sanctus, the acclamation after the Consecration, and the final Amen after the Per Ipsum. The Per Ipsum itself is reserved to the priest.